The invention relates to a multilayer inductor which has internal conductors of a main constituent being silver at the interior of the substrate thereof composed of a constituent belonging to spinel ferrite for attenuating noise elements, and a production method thereof.
In electronic machines, element mounting densities have kept on overcrowd, and there have been actualized mutual interference between elements or problems concerned with radiation of noises. Noise controlling means of many cases are higher harmonics of used signals, and attentions are paid to controlling of the higher harmonics. As the noise controlling means, there are multilayer inductors called as beads employing ferrite magnetic materials, means which shelters areas of circuit with metal plates to prevent bad influences to other circuit blocks, or means for avoiding spreading of noises to a next step by an LC resonance circuit.
Among them, according to the filter, a noise is not controlled, but the noise element is reflected to a front step and unexpected bad influences such as oscillation are given to the circuit, but since a multilayer inductor of noise absorption type does not cause such matters, it has been broadly used as a measure against noise.
A ceramic magnetic substance enabling to be sintered at around 900° C. and internal conductors composed of silver or its alloy are combined and baked together with the multilayer inductor so as to form coil shaped conductors at the interior of the ceramic sintered body. By shaping the internal conductors in coil, impedance may be made large, and loss of materials can be efficiently avoided, and as a result, the shape of the element can be made small sized.
Ferrite to be used to such purpose is called as spinel ferrite and it is almost composed of NiCuZnFe2O4 based component, and when using spinel ferrite, this kind of ferrite is sensitive to stress and an apparent permeability μ receives influence of stress and is remarkably lowered.
The multilayer inductor is baked together with silver powder for the internal conductors and ferrite powder for the substrate, and united in a one body. Since the silver is larger than the ferrite ceramic in coefficient of linear expansion, stress is, therefore, caused at the interface between silver and ferrite by baking together with, and the apparent μ of the ferrite is considerably lowered. Further, the interface therebetween is breakable, and the stress at the interface is released when heat-treating as soldering, so that problems occur that the apparent permeability μ is changed each time of the heat treatment and the characteristics are unstable.
For solving the problems, in JP-A-4-65807, a space 3 is defined as seen in FIG. 7 between the internal conductors 2 made of silver and the substrate 1 made of ferrite, thereby easing the stress exerting between the silver internal conductor 2 and the substrate 3 so as to improve the permeability μ. Numeral 4 designates external electrodes connected to both ends of the internal conductors.
However, as the multilayer inductor is based on a premise of the mounting by soldering, it is necessary to perform an electrolytic plating on the external electrodes 4, and a plating liquid goes into the space 3 between the internal conductor and the substrate. The plating liquid has an etching effect to ferrite and various bad influences to the ferrite substrate.